Mold flux for continuous-casting Ti-containing hypo-peritectic steel and method therefor

ABSTRACT

A mold flux is used in continuous casting of Ti-containing hypo-peritectic steel so as to prevent longitudinal cracks from forming on a surface of a slab. The mold flux contains CaO, SiO2, an alkali metal oxide and a fluorine compound as major components. f(1), f(2) and f(3), which are calculated from the initial chemical composition, are (1.1−0.5×T) to (1.9−0.5×T), 0.05 to 0.40 and 0 to 0.40, respectively, if the Ti content of the molten steel (mass %) is T. The TiO2 content in the melting state during the casting is no more than 20 mass % and the ratio of the first peak height of perovskite to the first peak height of cuspidine in the mold flux film is no more than 1.0.

TECHNICAL FIELD

This invention relates to mold flux that is used for continuous-castingTi-containing hypo-peritectic steel, and a method for continuous-castinghypo-peritectic steel containing 0.1 to 1 mass % of Ti, using the moldflux.

BACKGROUND ART

When hypo-peritectic steel containing 0.08 to 0.18 mass % of C iscontinuous-cast, a solidified shell that is formed by solidification ofmolten steel in a mold tends to be unequal in thickness, which causeslongitudinal cracks to easily form on a surface of a slab.

It is effective to mildly cooling the solidified shell (hereinafter, maybe referred to as “mild cooling”) in order to make the solidified shellin the mold equal in thickness. It is relatively easy to use mold fluxfor mild cooling.

The mold flux is supplied on molten steel in the mold, and melts withheat supplied from the molten steel. The mold flux in a melting stateflows along the mold, to come into a gap between the mold and thesolidified shell, and to form a mold flux film (hereinafter may bereferred to as “film”). Just after the casting starts, this film iscooled by the mold, to solidify like glass. Crystals are educed from theglass as time passes. When crystallization of this film is promoted, theroughness of the surface of the film in the mold side increases, whichcauses the thermal resistance at the interface between the mold and thefilm (hereinafter may be referred to as “interfacial thermalresistance”) to increase. Radiative heat transfer in the film is alsosuppressed. These effects allows the molten steel and the solidifiedshell touching the film to be mildly cooled down.

It is cuspidine (Ca₄Si₂O₇F₂) that is common composition of crystalseduced from the film.

The following methods have been worked out as means for promotingcrystallization of films:

A method for controlling fluid physical properties of mold flux,specifically, a method for raising the solidification point is effectivefor promoting crystallization of the film. About this method, PatentLiterature 1 describes that crystallinity of a film is improved byraising the solidification point of mold flux to the range of 1150 to1250° C. However, it is said there is a problem that if thesolidification point of the mold flux is raised to 1250° C. or over, thelubricity is disturbed and breakout cannot be prevented.

A method for controlling components in mold flux, specifically, a methodfor increasing the ratio of the contents of CaO to SiO₂ (hereinafter maybe referred to as “basicity”) is also effective for promotingcrystallization of the film. A method for reducing the MgO content inmold flux is also effective for promoting crystallization of the film.Concerning these methods, Patent Literature 2 discloses it is effectivefor crystallization of a film that in mold flux, the basicity isspecified by 1.2 to 1.6 and the MgO content is specified by no more than1.5 mass %. However, the highest temperature where the mold flux formscrystals disclosed in Patent Literature 2 is about 1150° C. at most, andonly an effect of mild cooling corresponding to this is obtained. Thatis, the effect of mild cooling is insufficient.

On the other hand, Patent Literature 3 discloses a method forsuppressing radiative heat transfer in a film by adding an iron oxide ora transition metal oxide to mold flux.

However, CaO, SiO₂, and CaF₂ in the mold flux are diluted by theaddition of any of these oxides. Specifically, in Patent Literature 3,no less than 10 mass % of an iron oxide or a transition metal oxide intotal has to be added as shown in its Examples in order to obtain asufficient effect of suppressing radiative heat transfer. In this case,cuspidine is difficult to be educed when the composition has about 1.0of the basicity shown in Examples of Patent Literature 3, and thesolidification point of the mold flux drops. The solidification pointshown in Examples of Patent Literature 3 is about 1050° C., which islower than that in Patent Literature 1 by no less than 100° C.considering that the solidification point of the mold flux forhypo-peritectic steel suggested in Patent Literature 1 is about 1150 to1250° C. As a result, crystallization of the film is blocked. Thus, withthe art of Patent Literature 3, an effect of mild cooling obtained fromincrease of interfacial thermal resistance and the like according to thecrystallization is marred.

Patent Literature 4 discloses a range of the composition of mold flux ofthe quaternary system of CaO—SiO₂—CaF₂—NaF where cuspidine is easilyeduced. The range of the composition is substantially same as a primarycrystallization field of cuspidine as published in Non-Patent Literature1 thereafter. According to the mold flux of Patent Literature 4 asdescribed above, longitudinal cracks do not form on a surface of a slabwhen hypo-peritectic steel is rapidly cast, which makes it possible toobtain the slab which has a good surface quality.

Patent Literature 5 discloses a method for adding a transition metaloxide to the basic composition prepared within the range of PatentLiterature 4, to drop the solidification point without marring an effectof mild cooling. Patent Literature 5 is aimed at the problem that whenthe Mn content in molten steel is high, crystallization of cuspidine isblocked because the MnO content in the film increases due to oxidationreaction of Mn, and thus the effect of mild cooling cannot besufficiently obtained. For this problem, a necessary content of MnO iscontained in advance, to suppress oxidation reaction of Mn, and then thesolidification point is raised to a desired level. Whereby, it ispossible to prevent longitudinal cracks on high-strength steel of thehigh Mn content from forming, according to Patent Literature 5.

CITATION LIST Patent Literature

Patent Literature 1: JP H8-197214A

Patent Literature 2: JP H8-141713A

Patent Literature 3: JP H7-185755A

Patent Literature 4: JP2001-179408A

Patent Literature 5: JP2006-289383A

Non-Patent Literature

Non-Patent Literature 1: ISIJ International, Vol. 42 (2002), pp. 489 to497

SUMMARY OF INVENTION Technical Problem

As described above, in continuous casting of hypo-peritectic steel,longitudinal cracks easily form on a surface of a slab. It is effectivefor preventing the formation of longitudinal cracks to carry out mildcooling on the solidified shell, and mold flux can be used for this mildcooling.

However, the mold flux of Patent Literatures 1 to 3 as described abovehas the problems that the lubricity is disturbed and breakout cannot beprevented, and that the effect of mild cooling is insufficient.

On the other hand, according to the mold flux of Patent Literature 4,longitudinal cracks do not form on a surface of a slab whenhypo-peritectic steel is rapidly cast, which makes it possible to obtainthe slab which has a good surface quality. According to the mold flux ofPatent Literature 5, it is possible to prevent longitudinal cracks onhigh-strength steel of the high Mn content from forming.

One of steel grades of hypo-peritectic steel is of no less than 0.1 mass% of the Ti content. In casting of this Ti-containing hypo-peritecticsteel, TiO₂ forms in mold flux in a melting state through the influenceof oxidation reaction of Ti in molten steel. This TiO₂ not only dilutescuspidine in the solidified film, but also forms another new crystalphase, perovskite (CaTiO₃). Therefor, this perovskite grows up in thefilm unilaterally, and a glass phase (cuspidine) necessary forlubrication is marred. As a result, stable casting gets difficult, andthe problem of forming longitudinal cracks on a surface of a slab rises.

Therefore, there is a case in the casting of Ti-containinghypo-peritectic steel that longitudinal cracks form on a surface of aslab through the influence of TiO₂ forming in the mold flux even if themold flux of any of Patent Literatures 4 and 5 is used.

This invention was made in view of these problems. An object of thisinvention is, in continuous casting of Ti-containing hypo-peritecticsteel, to provide mold flux that can prevent longitudinal cracks fromforming on a surface of a slab and to provide a method forcontinuous-casting hypo-peritectic steel containing 0.1 to 1 mass % ofTi, using this mold flux.

Solution to Problem

The inventors of this invention found that in continuous casting ofTi-containing hypo-peritectic steel, the composition of mold flux in amelting state changes according to oxidation reaction of Ti in moltensteel. Specifically, they found that the MnO and TiO₂ contents of themold flux increase in its melting state from less than 0.1 mass % in itsinitial composition.

Further, they found that even if f(1), f(2) and f(3) described below,which are calculated from the initial composition of the mold flux,satisfy the formulas (1), (2) and (3) described below as well,composition changes in the mold flux in the melting state become largewhen the TiO₂ content of the mold flux in the melting state duringcasting exceeds 20 mass %. If the composition changes in the mold fluxin the melting state become large, the ratio of the first peak height ofperovskite to the first peak height of cuspidine (hereinafter may bemerely referred to as “strength”) which is obtained from X-raydiffraction analysis of powder obtained by pulverizing the film of themold flux in a solidifying state after the casting takes a value morethan 1.0, formation of cuspidine is blocked, and evaluation ofcontinuous casting and longitudinal cracks becomes “failure”. Thus, itis important for preventing longitudinal cracks on a surface of a slabfrom forming in continuous casting of Ti-containing hypo-peritecticsteel that the TiO₂ content of mold flux in the melting state during thecasting is less than 20 mass % and the strength ratio is no more than1.0. This invention was made based on these findings. The gist of thisinvention is as follows.

A first aspect of this invention is mold flux for continuous-castingTi-containing hypo-peritectic steel, wherein in continuous casting ofTi-containing hypo-peritectic steel, the mold flux contains CaO, SiO₂,an alkali metal oxide and a fluorine compound as major components,chemical composition of the mold flux before the mold flux is put into amold satisfies the formulas (1), (2) and (3), a TiO₂ content of the moldflux in a melting state during the casting is no more than 20 mass %,and a strength ratio of a film of the mold flux in a solidifying stateafter the casting is no more than 1.0:1.1−0.5×T≤f(1)≤1.9−0.5×T  (1)0.05≤f(2)≤0.40  (2)0≤f(3)≤0.40  (3),

wherein in the formulas (1) to (3),f(1)=(CaO)_(h)/(SiO ₂)_(h)  (A)f(2)=(CaF ₂)_(h)/{(CaO)_(h)+(SiO ₂)_(h)+(CaF ₂)_(h)}  (B)f(3)={(alkali metal fluoride)_(h)}/{(CaO)_(h)+(SiO ₂)_(h)+(alkali metalfluoride)_(h))}  (C),

wherein in the formulas (A) to (C),(CaO)_(h) =W _(CaO)−(CaF ₂)_(h)×0.718  (D)(SiO ₂)_(h) =W _(SiO2)  (E)(CaF ₂)_(h)=(W _(F) −W _(Li2O)×1.27−W _(Na2O)×0.613−W_(K2O)×0.403)×2.05  (F)(alkali metal fluoride)_(h) =W _(Li2O)×1.74+W _(Na2O)×1.35+W_(K2O)×1.23  (G)

wherein T is a Ti content of molten steel (mass %), W_(CaO) is a CaOcontent of the mold flux (mass %), W_(SiO2) is a SiO₂ content of themold flux (mass %), W_(F) is a F content of the mold flux (mass %), andW_(Li2O), W_(Na2O) and W_(K2O) are contents of Li₂O, Na₂O and K₂Orespectively, which are alkali metal oxides, of the mold flux (mass %),and

wherein the strength ratio of the film means a ratio of a first peakheight of perovskite (strength of an angle (33.2°), which is twice aswide as a Bragg angle when Co was a source, X2) to a first peak heightof cuspidine (strength of an angle (29.2°), which is twice as wide as aBragg angle when Co was a source, X1), the ratio (X2/X1) being obtainedfrom X-ray diffraction analysis of powder obtained by pulverizing thefilm of the mold flux.

A second aspect of this invention is a method for continuous-castingTi-containing hypo-peritectic steel, the method comprising:continuous-casting hypo-peritectic steel containing 0.1 to 1 mass % ofTi, using the mold flux of the above first aspect of this invention.

“ . . . contains CaO, SiO₂, an alkali metal oxide and a fluorinecompound as major components” in this invention means that the contentof each object component is no less than 5 mass %, and the total contentthereof is no less than 70 mass %.

Advantageous Effects of Invention

Each of the indexes (f(1), f(2) and f(3)) of the mold flux forcontinuous-casting Ti-containing hypo-peritectic steel of this invention(hereinafter may be referred to as “mold flux of this invention”) isprepared within a predetermined range; these indexes are calculated fromthe chemical composition before the mold flux is supplied into a mold(hereinafter may be referred to as “initial chemical composition”).Moreover, the TiO₂ content in the melting state during the casting is nomore than 20 mass % and the strength ratio of the film in thesolidifying state after the casting is no more than 1.0. Whereby, evenif the composition of the mold flux in the melting state changesaccording to oxidation reaction of Ti in molten steel, cuspidinestabilizes in a crystal phase in the film, and a state where cuspidineis dominant over perovskite can be kept. As a result, effects oflubricity and mild cooling in the mold are stable, to preventlongitudinal cracks on a surface of a slab from forming.

The method for continuous-casting Ti-containing hypo-peritectic steel ofthis invention (hereinafter may be referred to as “continuous castingmethod of this invention”) uses the above described mold flux of thisinvention. Whereby, cuspidine stabilizes in a crystal phase in the filmthat is formed in the mold, and the state where cuspidine is dominantover perovskite can be kept. As a result, the effects of lubricity andmild cooling in the mold are stable, to prevent longitudinal cracks on asurface of a slab from forming.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing the mold flux and the continuous casting methodof this invention.

FIG. 2 is a cross-sectional view showing partially enlarged FIG. 1.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is a view showing this invention. FIG. 2 is a cross-sectionalview showing a part of FIG. 1 surrounded by a dashed line enlarged. Thisinvention will be described below with reference to FIGS. 1 and 2 whendemanded. “X to Y” means “no less than X and no more than Y” unlessthere is any special mention.

As shown in FIG. 1, mold flux 1 of this invention is supplied on thesurface of molten steel 4 that is poured into a mold 3 via a submergednozzle 2. The mold flux 1 of this invention supplied in this way meltswith heat supplied from the molten steel 4. After that, as shown in FIG.2, the mold flux 1 flows along the mold 3, and comes into a gap betweenthe mold 3 and a solidified shell 5, to form a film 8. The solidifiedshell 5, which is formed by cooling from the side of the mold 3 that iscooled by cooling means not shown, is withdrawn toward a lower part ofthe mold 3 with rolls 6, and is cooled by cooling water 7. In thecontinuous casting method of this invention, hypo-peritectic steelcontaining 0.1 to 1 mass % of Ti is continuous-cast in this way.

The reasons why the mold flux and continuous casting method of thisinvention are specified like the above, and preferred embodimentsthereof will be described below.

The mold flux of this invention contains CaO, SiO₂, an alkali metaloxide and a fluorine compound as major components. CaO, SiO₂ and afluorine compound are contained as essential components for cuspidinethat bears crystallization. An alkali metal oxide is contained as acomponent for controlling the solidification point of the fluxrelatively easily.

As described above, in the continuous casting of hypo-peritectic steelcontaining 0.1 to 1 mass % of Ti, the chemical composition of the moldflux changes according to oxidation reaction of Ti in the molten steelin the mold. Thus, each of the indexes f(1), f(2) and f(3), hereinafterthe same will be referred to) of the mold flux of this invention, whichis calculated from the initial chemical composition, is prepared withina predetermined range. Here, “initial chemical composition” means thecomposition before the supply into the mold for continuous casting. Theintention is to exclude the composition changes in the mold fluxaccording to oxidation reaction of Ti in the molten steel.

The preparation of the indexes makes cuspidine stabilize in a crystalphase in the film even if the composition of the mold flux in themelting state (hereinafter may be referred to as “melting mold flux”)changes according to oxidation reaction of Ti in the molten steel. Thus,a state where cuspidine is dominant over perovskite is easily kept. As aresult, the effects of lubricity and mild cooling in the mold can bestable, and longitudinal cracks on a surface of a slab can be preventedfrom forming.

Specifically, the initial chemical composition satisfies the followingformulas (1), (2) and (3). That is, the indexes (f(1), f(2) and f(3)),which are calculated from the initial chemical composition using thefollowing formulas (A) to (H), satisfy the following formulas (1), (2)and (3), respectively.1.1−0.5×T≤f(1)≤1.9−0.5×T  (1)0.05≤f(2)≤0.40  (2)0≤f(3)≤0.40  (3),

The indexes f(1) to f(3) are specified by the following formulas (A) to(G).f(1)=(CaO)_(h)/(SiO ₂)_(h)  (A)f(2)=(CaF ₂)_(h)/{(CaO)_(h)+(SiO ₂)_(h)+(CaF ₂)_(h)}  (B)f(3)={alkali metal fluoride)_(h)}/{(CaO)_(h)+(SiO ₂)_(h)+(alkali metalfluoride)_(h))}  (C)(CaO)_(h) =W _(CaO)−(CaF ₂)_(h)×0.718  (D)(SiO ₂)_(h) =W _(SiO2)  (E)(CaF ₂)_(h)=(W _(F) −W _(Li2O)×1.27−W _(Na2O)×0.613−W_(K2O)×0.403)×2.05  (F)alkali metal fluoride)_(h) =W _(Li2O)×1.74+W _(Na2O)×1.35+W_(K2O)×1.23  (G)

Here T is the Ti content in the molten steel (mass %), W_(CaO) is theCaO content in the mold flux (mass %), W_(SiO2) is the SiO₂ content inthe mold flux (mass %), W_(F) is the F content in the mold flux (mass%), and W_(Li2O), W_(Na2O) and W_(K2O) are the contents of Li₂O, Na₂Oand K₂O respectively, which are alkali metal oxides, in the mold flux(mass %).

The index f(1), which is calculated using the formula (A), is a ratio ofthe CaO content to the SiO₂ content in view of CaF₂, and is an importantindex to promote crystallization of cuspidine.

Here, in the case of hypo-peritectic steel whose Ti content is less than0.1 mass %, the value of f(1) has to take 1.1 to 1.9 in order to keepthe composition of the melting mold flux within the range of thecomposition of a primary crystal of cuspidine.

In the case of hypo-peritectic steel containing 0.1 to 1 mass % of Ti,SiO₂ in the melting mold flux is reduced by reaction with Ti in themolten steel in the mold. Therefore, such a situation arises that evenif f(1) from the initial chemical component is within the abovedescribed range (1.1 to 1.9), the value of f(1) from the composition ofthe melting mold flux is far beyond the preferred state. Thus, f(1) fromthe initial chemical composition is prepared so as to be low accordingto the Ti content of the molten steel, and thereby the value of f(1)from the composition of the melting mold flux is set within the abovedescribed range (1.1 to 1.9). As a result, the value of f(1) from thecomposition of the melting mold flux increases because of the reactionin the mold, and the composition of the melting mold flux can be keptwithin the range of the composition of a primary crystal of cuspidine.

Specifically, f(1) of the mold flux of this invention has to be(1.1−0.5×T) to (1.9−0.5×T). In view of more stable crystallization ofcuspidine, the upper limit of f(1) is preferably (1.7−0.5×T), and morepreferably (1.5−0.5×T). In the same view, the lower limit of f(1) ispreferably (1.2−0.5×T), and more preferably (1.3−0.5×T).

The index f(2) calculated using the formula (B) indicates a proportionof CaF₂ for the total content of CaO, SiO₂ and CaF₂, and is an importantindex to promote crystallization of cuspidine. Setting f(2) in 0.05 to0.40 makes it possible to keep the composition of the melting mold fluxwithin the range of the composition of a primary crystal of cuspidine.In view of more stable crystallization of cuspidine, the upper limit off(2) is preferably 0.3, and more preferably 0.25. In the same view, thelower limit of f(2) is preferably 0.1, and more preferably 0.15.

The index f(3) calculated using the formula (C) indicates a proportionof a component that plays a role like a solvent for cuspidine. Settingf(3) in no more than 0.4 makes it possible to keep crystallization ofcuspidine. The lower limit of f(3) is 0 according to the definition ofthe formula (C). In view of more stable crystallization of cuspidine,the upper limit of f(3) is preferably 0.20, and more preferably 0.15. Inthe same view, the lower limit of f(3) is preferably 0.05, and morepreferably 0.10.

The f(1), f(2) and (3) of the mold flux of this invention from theinitial chemical composition satisfy the formulas (1), (2) and (3),respectively. Whereby, even if the composition changes according toreaction with the molten steel, cuspidine stabilizes in a crystal phasein the film, and the state where cuspidine is dominant over perovskitecan be kept.

The TiO₂ content of the mold flux of this invention in the melting statein the casting is no more than 20 mass %, and the strength ratio of thefilm of the mold flux of this invention in the solidifying state afterthe casting is no more than 1.0. No more than 20 mass % of the TiO₂content of the melting mold flux makes it possible to suppresscomposition changes in the melting mold flux. Thus, cuspidine stabilizesin a crystal phase in the film, and the state where cuspidine isdominant over perovskite can be kept. No more than 1.0 of the strengthratio of the film of the mold flux in the solidifying state after thecasting makes it possible not to block formation of cuspidine. No morethan 20 mass % of the TiO₂ content of the melting mold flux and no morethan 1.0 of the strength ratio of the film of the mold flux in thesolidifying state after the casting in addition to the satisfaction ofthe formulas (1), (2) and (3) makes it possible to prevent longitudinalcracks from forming on a surface of a slab.

The solidification point of the mold flux is preferably 1150 to 1400° C.If the solidification point is under 1150° C., crystallization ofcuspidine might be poor. It is technically difficult to make thesolidification point over 1400° C. The solidification point of 1150 to1400° C. improves the effect of mild cooling by film. Thus, longitudinalcracks can be surely prevented from forming.

The viscosity of the mold flux is preferably no more than 2 poises (=0.2Pa·s) at 1300° C. If the viscosity is over 2 poises, the crystallizationrate might be down. If the viscosity is no more than 2 poises, theeffect of mild cooling by film is improved and longitudinal cracks canbe surely prevented from forming. On the other hand, concerning thelower limit of the viscosity, there arises no problem due to lowviscosity. However, it is difficult to make the viscosity of generallyused mold flux less than 0.1 poise (=0.01 Pa·s). Thus, no less than 0.1poise is preferable.

When the Ti content of hypo-peritectic steel is no less than 0.1 mass %,the problem is outstanding that longitudinal cracks form on a surface ofa slab through the influence of oxidation reaction of Ti in the steel.In contrast, when the Ti content of hypo-peritectic steel exceeds 1 mass%, composition changes in the melting mold flux in the mold through theinfluence of oxidation reaction of Ti in the molten steel become large.As a result, it becomes difficult to keep the composition of the meltingmold flux within the range of the composition of a primary crystal ofcuspidine. Therefore, the hypo-peritectic steel continuous-cast usingthe mold flux of this invention is specified as hypo-peritectic steelcontaining 0.1 to 1 mass % of Ti.

In this invention, for example, at least one of Li₂O, Na₂O and K₂O canbe used as an alkali metal oxide. For example, fluorite that containsCaF₂ as a major component, or NaF can be used as a fluorine compound.

In addition, Al₂O₃ may be contained in the mold flux of this inventionin order to adjust physical properties such as the solidification pointand the viscosity. Al₂O₃ has the functions of dropping thesolidification point and increasing the viscosity. However, the Al₂O₃content is preferably low in order to promote crystallization ofcuspidine. The Al₂O₃ content is preferably no more than 5 mass %. Incontrast, when general raw materials for mold flux are used, about 0.5mass % or more of Al₂O₃ is inevitably contained therein. While the Al₂O₃content can be less than 0.5 mass % by using artificial raw materialslike a pre-melting base material, it might be accompanied by a rise inraw material costs. Therefore, the Al₂O₃ content is preferably no lessthan 0.5 mass %.

The continuous casting method of this invention is directed tohypo-peritectic steel containing 0.1 to 1 mass % of Ti. The method usesthe above described mold flux of this invention as mold flux. Whereby,the composition of a crystal phase in the film formed in the mold ismaintained during the casting. That is, the state where cuspidine isdominant over perovskite in a crystal phase in the film can be keptduring the casting. Thus, the effects of lubricity and mild cooling inthe mold can be stable, and longitudinal cracks on a surface of a slabcan be prevented.

The continuous casting method of this invention has no specificlimitation to the casting conditions other than the mold flux. That is,the casting conditions can be properly set as well as a conventionalcontinuous casting method.

EXAMPLES

For confirming effects of the mold flux and continuous casting method ofthis invention, continuous casting tests were carried out and resultsthereof were evaluated.

In these tests, a slab was continuous-east from 2.5 ton of molten steelwhile the mold flux was supplied onto the molten steel in a mold. Atthis time, the withdrawal rate was 1.0 m/min, and the size of the slabwas: 500 mm in width, 84 mm in thickness and 7000 mm in length.

Table 1 shows grades (symbol), initial chemical composition (mass %),basicities, solidification points (° C.) and viscosities (poise) at1300° C. of the mold flux used for the tests. Table 2 shows the chemicalcomposition (mass %) of the molten steel used for the tests.

TABLE 1 Mold Flux Grade Initial Chemical Composition (mass %) BasicitySolidification Viscosity (Symbol) SiO₂ CaO Al₂O₃ MgO Na₂O MnO TiO₂ F (—)Point (° C.) (poise) A1 29.2 49.7 3.9 0.6 5.8 <0.1 <0.1 10.8 1.7 12450.5 A2 33.5 46.8 2.5 0.5 6.1 <0.1 <0.1 10.6 1.4 1228 0.7 R1 33.8 39.57.2 0.6 10.8 <0.1 <0.1 8.1 1.2 1210 2.3 R2 38.0 38.0 2.8 0.4 12.8 <0.1<0.1 8.0 1.0 1190 1.5 R3 39.2 27.4 2.2 0.5 19.0 <0.1 <0.1 10.0 0.7 10501.5

TABLE 2 Chemical Composition of Molten Steel (unit: mass %) Remainder:Fe and Impurities C Si Mn P S Ti Al 0.09-0.11 0.10-0.20 1.30-1.400.010-0.015 0.002-0.005 0.1-1.2 0.01-0.04

Test numbers 1 to 7 were set in the tests. The grade of the mold fluxand the chemical composition of the molten steel were changed in eachtest. Table 3 shows grades of the mold flux, the Ti contents in themolten steel (mass %), values of f(1), f(2) and f(3) calculated usingthe initial chemical composition (hereinafter may be referred to as“initial composition”), and test categories used in the tests.

TABLE 3 Mold Flux Ti Content in f(1) from f(2) from f(3) from TestMolten Steel Initial Initial Initial Test Number (mass %) GradeComposition Composition Composition Category 1 0.19 A1 1.34 0.18 0.10Ex. of This Invention 2 0.42 A1 1.34 0.18 0.10 Ex. of This Invention 30.41 A2 1.10 0.06 0.10 Ex. of This Invention 4 1.12 A1 1.34 0.18 0.10Comp. Ex. 5 0.20 R1 1.11 0.04 0.17 Comp. Ex. 6 0.18 R2 0.99 0 0.19 Comp.Ex. 7 0.19 R3 0.70 0 0.28 Comp. Ex.

In each test, the mold flux in the melting state was taken out of themold during the casting, and its components were analyzed. Table 4 showsthe chemical composition of the mold flux in the melting state, andvalues of f(1), f(2) and f(3) calculated using the composition in themelting state.

TABLE 4 Mold Flux f(1) in f(2) in f(3) in Test Chemical Composition inMelting State (mass %) Melting Melting Melting Number Grade SiO₂ CaOAl₂O₃ MgO Na₂O MnO TiO₂ F State State State 1 A1 24.1 47.1 5.4 0.5 5.61.3 5.8 10.2 1.5 0.18 0.11 2 A1 19.8 45.6 6.8 0.5 5.7 1.4 9.7 10.5 1.80.21 0.12 3 A2 21.6 43.2 7.1 0.4 5.8 1.3 10.2 10.4 1.5 0.20 0.13 4 A113.0 37.7 10.3 0.5 5.2 1.6 21.9 9.8 2.2 0.25 0.15 5 R1 18.1 41.6 4.5 0.511.1 2.1 13.6 8.5 2.2 0.06 0.21 6 R2 17.7 35.8 8.6 0.4 11.4 1.9 17.1 7.12.0 0.00 0.22 7 R3 25.4 26.6 4.9 0.5 16.2 2.3 13.6 10.5 1.0 0.02 0.30

The film in a solidifying state was taken out of the mold when thecasting was ended, and pulverizing was carried out on the film to obtainpowder. The obtained powder underwent X-ray diffraction analysis. Fromthe results of the diffraction analysis, the strength of cuspidine andthe strength of perovskite were obtained, to calculate the ratio (X2/X1)of the strength of perovskite (X2) to the strength of cuspidine (X1). Atthis time, the strength of cuspidine was the first peak height,specifically, the strength of an angle (29.2°), which was twice as wideas a Bragg angle when Co was a source. The strength of perovskite wasthe first peak height, specifically, the strength of an angle (33.2°),which was twice as wide as a Bragg angle when Co was a source.

Longitudinal cracks on a surface of a slab was checked. In this check, asurface of a cast slab was observed visually, and the length of anobserved crack in a longitudinal direction was measured. At this time,if a crack of no less than 10 mm was detected, it was determined to formlongitudinal cracks. In addition, temperature of a copper plate of themold was measured upon continuous casting, and its temperature changewas observed. From them, continuous casting and longitudinal cracks wereevaluated for each test.

The symbols in the “Evaluation of Continuous Casting and LongitudinalCracks” column in Table 5 represent the following:

∘: represents that the temperature of a copper plate of the mold wasstable upon continuous casting, the continuous casting was able to becompleted, and no longitudinal crack formed on a surface of the castslab; that is, “excellent”.

Δ: represents that while the temperature of a copper plate of the moldchanged upon continuous casting, the continuous casting was able to becompleted, and longitudinal crack formed on a surface of the cast slab;that is, “failure”.

x: represents that the temperature of a copper plate of the moldconsiderably changed upon continuous casting, and the continuous castingwas stopped in the middle; that is, “failure”.

Table 5 shows test numbers, grades of the mold flux, the Ti contents(mass %) in the molten steel, the ratio of the strength of perovskite tothe strength of cuspidine (strength ratio) and evaluations of continuouscasting and longitudinal cracks.

TABLE 5 Ti Content in Evaluation of Test Grade of Molten Steel StrengthContinuous Casting and Number Mold Flux (mass %) Ratio LongitudinalCracks 1 A1 0.19 0.6 ∘ 2 A1 0.42 0.8 ∘ 3 A2 0.41 0.6 ∘ 4 A1 1.12 1.5 x 5R1 0.20 2.1 x 6 R2 0.18 1.6 Δ 7 R3 0.19 1.2 Δ

As seen from Tables 1 to 5, each MnO and TiO₂ content of the mold fluxof all the test numbers 1 to 7 was less than 0.1 mass % in the initialcomposition. On the other hand, in the melting state, each MnO and TiO₂content increased. From these results, it was confirmed that in thecontinuous casting of Ti-containing hypo-peritectic steel, thecomposition of the mold flux in the melting state changed according tooxidation reaction of Ti in the molten steel.

The index f(2) of the mold flux used in the test number 5, which wascalculated from the initial composition, did not satisfy the formula(2). The indexes f(1) and f(2) of the mold flux used in the test numbers6 to 7, which were calculated from the initial chemical composition, didnot satisfy the formulas (1) and (2), respectively. As a result, in eachtest number 5 to 7, the strength ratio of the film took a value largerthan 1.0, that is, formation of cuspidine was blocked. Therefore, theevaluation of continuous casting and longitudinal cracks was “failure”.

In contrast, f(1), f(2) and f(3) of the mold flux used in each testnumber 1 to 3, which were calculated from the initial composition,satisfied the formulas (1), (2) and (3), respectively. In addition, theTiO₂ content of the melting mold flux was no more than 20 mass %, andthe strength ratio of the film was less than 1.0. As a result, a statewhere cuspidine was dominant over perovskite was kept during the castingin each test number 1 to 3. Therefore, the evaluation of continuouscasting and longitudinal cracks was “excellent”.

The indexes f(1), f(2) and f(3) of the mold flux used in the test number4, which were calculated from the initial composition, satisfied theformulas (1), (2) and (3), respectively. However, in the test number 4,the Ti content of the molten steel was over 1.0 mass %, and thus theTiO₂ content of the melting mold flux was over 20 mass %. Thus, thecomposition change in the melting mold flux was large. As a result, thestrength ratio of the film took a value larger than 1.0, that is,formation of cuspidine was blocked. Therefore, the evaluation ofcontinuous casting and longitudinal cracks was “failure”.

From these results, it was made clear that according to the mold fluxand the continuous casting method of this invention, the state wherecuspidine was dominant over perovskite was able to be kept in a crystalphase of the film, and longitudinal cracks on a surface of a slab wasable to be prevented.

While this invention has been described concerning the embodiments thatare considered to be the most practical and preferable at present, thisinvention is not limited to the embodiments disclosed in thisdescription, and can be properly modified within the scope not contraryto the gist and ideas of this invention readable from the claims andwhole of the description, and it should be understood that mold flux forcontinuous-casting Ti-containing hypo-peritectic steel and a continuouscasting method with such modification are also encompassed within thetechnical range of this invention.

INDUSTRIAL APPLICABILITY

According to the mold flux and the continuous casting method of thisinvention, the effect of lubricity and mild cooling in the mold isstable, and longitudinal cracks on a surface of a slab can be preventedfrom forming. Thus, they can be effectively used in continuous castingof hypo-peritectic steel containing 0.1 to 1 mass % of Ti.

REFERENCE SIGNS LIST

-   1 . . . mold flux for continuous-casting Ti-containing    hypo-peritectic steel-   2 . . . submerged nozzle-   3 . . . mold-   4 . . . molten steel-   5 . . . solidified shell-   6 . . . rolls-   7 . . . cooling water-   8 . . . film

The invention claimed is:
 1. A method for continuous-castingTi-containing hypo-peritectic steel, the method comprising:continuous-casting hypo-peritectic steel containing 0.1 to 1 mass % ofTi, using a mold flux, wherein the mold flux contains CaO, SiO₂, analkali metal oxide and a fluorine compound as major components, achemical composition of the mold flux before the mold flux is put into amold satisfies the formulas (1), (2) and (3), a TiO₂ content of the moldflux in a melting state during the casting is no more than 20 mass %,and a strength ratio of a film of the mold flux in a solidifying stateafter the casting is no more than 1.0:1.1−0.5×T≤f(1)≤1.9−0.5×T  (1)0.05≤f(2)≤0.40  (2)0≤f(3)≤0.40  (3), wherein in the formulas (1) to (3),f(1)=(CaO)_(h)/(SiO ₂)_(h)  (A)f(2)=(CaF ₂)_(h)/{(CaO)_(h)+(SiO ₂)_(h)+(CaF ₂)_(h)}  (B)f(3)={(alkali metal fluoride)_(h)}/{(CaO)_(h)+(SiO ₂)_(h)+(alkali metalfluoride)_(h))}   (C), wherein in the formulas (A) to (C),(CaO)_(h) =W _(CaO)−(CaF ₂)_(h)×0.718  (D)(SiO ₂)_(h) =W _(SiO2)  (E)(CaF ₂)_(h)=(W _(F) −W _(Li2O)×1.27−W _(Na2O)×0.613−W_(K2O)×0.403)×2.05  (F)(alkali metal fluoride)_(h) =W _(Li2O)×1.74+W _(Na2O)×1.35+W_(K2O)×1.23  (G) wherein T is a Ti content of molten steel (mass %),W_(CaO) is a CaO content of the mold flux (mass %), W_(SiO2) is a SiO₂content of the mold flux (mass %), W_(F) is a F content of the mold flux(mass %), and W_(Li2O), W_(Na2O) and W_(K2O) are contents of Li₂O, Na₂Oand K₂O respectively, which are alkali metal oxides, of the mold flux(mass %), and wherein the strength ratio of the film means a ratio of afirst peak height of perovskite to a first peak height of cuspidine, theratio being obtained from X-ray diffraction analysis of powder obtainedby pulverizing the film of the mold flux.